California almonds [Prunus dulcis, syn. P. amygdalus, P. communis] possess a moderately lignified `paper' shell rather than the stony, peach-pit type shells common to European and Asian cultivars. At nut maturity, more than 70% of shells of the principal California cultivar Nonpareil can be split. Use of a mechanical shaker to harvest nuts increased the proportion of nuts with split shells by 40% when compared to hand harvest. All shell splitting occurred at the ventral suture with ∼ 80% of the splits occurring at the site of the degenerating funiculus leading to the aborted or secondary ovule. Remaining splits occurred near the site of the funiculus feeding the viable ovule, and only rarely at the suture line. Abortion of one of the two ovules in the almond ovary is often initiated at or shortly after bloom, and so the final site of shell splitting appears to be predetermined early in fruit development. Measurements of the strength of the inner endocarp wall at 50 days after flowering showed distinct weaknesses in the areas of the developing funiculi. Similarly, damage to the developing kernel at 60 days after flowering by the leaffooted bug (Leptoglossus clypealis Heiderman) occurred along the ventral suture, with 80% of the damage located at the point of attachment of the secondary funiculus.
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